An endemic focus of Wuchereria bancrofti in Mountain Province, Luzon (Calaccad Valley), is discussed. A 10.7% microfilaremia rate was found. The parasites were probably brought into this area in the original human migration, as recent immigrants came from non-infected areas. No infection was found in children under 12 years of age, which correlates with spraying of the area since 1953. However, some people live in unsprayed houses, and children may become infected. With coming road development and urbanization, Culex p. fatigans will move in and undoubtedly increase transmission. Currently Anopheles minimus flavirostris is the most important local vector. Aedes (Finlaya) niveus is probably also a vector.